The present invention relates to pellet mill dies, more particularly to pellet mill dies which have tapered grooves along each side of the compression face of the die.
In pellet mills, the mixture of material to be pelleted is normally fed to a die having a plurality of extrusion holes therethrough. An extrusion means, generally one or more extrusion rolls, travels over the compression side of the die and squeezes the material between the die and the rolls. This movement forces the material through the die holes. As the material emerges from the discharge side of the die, the extrusions are severed to produce pellets.
Generally, the dies of the prior art are annular or circular in shape although they may be in the shape of a horizontal plane. The dies have a compression side and a discharge side with a plurality of die holes of suitable shape preferably arranged in rows and extending through the die from the compression side to the discharge side. When in rows, the holes will preferably be staggered with respect to holes in adjacent rows as is conventional.
In cooperation with the die are one or more extrusion rolls each of which spans the rows of die holes and traverses the compression side of the die. Material deposited on the surface of the die is extruded through the die holes and expelled from the discharge side of the die. As the extrusion rolls force the material through the holes, the compression surface of the die is worn. Since the rollers are generally wider than the die face, without some form of relief it would be necessary for the roller to grind away the area of the die outside the outer rows of die holes. This would cause severe wear of the outer edges of the rollers.
In order to overcome this problem, it has become somewhat standard practice to have grooves or channels along either side of the compression face of the die. The grooves are located along each side of the rows of die holes so that the ends of the compression rollers are positioned over the grooves. See for example, U.S. Pat. No. 3,129,458. The primary function of the grooves is to make provisions for the outer edges of the compression rollers as the die face wears. Since the distance between the outer diameter of the extrusion rollers and the die face must be maintained throughout the life of the die, the roller assembly is adjustable outwardly. Since the rollers are wider than the die face, the grooves provide a material free place for their outer edges. Without this relief it would be necessary for the rollers to physically grind away this area of the die as the die wears under normal usage. This contact stress would be quite high with resultant severe wear of the outer edges of the extrusion rollers.
Although the use of grooves has become acceptable practice and solves the problem of excessive wear of the extrusion rollers it is not without drawbacks. For example, the grooves of the prior art are U-shaped or channel shaped. Under normal wear conditions the die face wears in such a manner as to produce a ridge of metal between the outer rows of holes and the grooves which stands up and is subject to much stress. Fatigue failure can be experienced in this area of the die causing breakage along the edge of the groove. Such breakage greatly reduces the effectiveness of the outer rows of holes causing early replacement of the die.
It is the object of the present invention to provide a pellet mill die with prolonged life and reduced tendency for fatigue failure.